Rock bolts are principally used in mining and tunneling.
A rock bolt generally is introduced into a borehole and may be grouted in place, an end of the anchor extending out of the hole and being threaded to accommodate a nut which is braced against a bearing plate.
The grouting may be any hardenable mass, e.g. a synthetic resin and usually a two-component synthetic resin which can be provided in a cartridge which is disrupted by the insertion of the anchor. The latter may be rotated to mix the two components and then is merely permitted to be locked in place in the hardening mass.
A rigid rock bolt, anchor or similar stabilizer provides for reinforcement of the rock or other structure due to the low yielding properties or characteristics of the anchor, and in conjunction with a full grouting because of the strong bond with the rock. The reinforcement action is, however, reduced or eliminated when stress-release movements arise i.e. expansion or contraction or shifting sets in to crack the structure around the anchor
However, one constantly has to expect stress-release or stress-shifting movements of the rock, particularly in the case of underground mining or tunneling. When these movements occur the anchors lose their value, and use must be made of expansion or yielding types of rock bolts or anchors to compensate for such movement. This latter type of anchor allows adaptation of the so-called rock armoring to movements of the surrounding rock matter or the like environment, due to the definable expansion or tensile properties of the rock bolt.
Such adaptation to the respective movements of the host environment or rock is only then feasible with an anchor rod when it is made of a material which has corresponding elastic, expanding or, in the most general sense, yielding properties. For this adaptation, furthermore, the anchor rod is securely clamped in place or tensioned at the toe end of the borehole by being securely cemented at the base of the borehole. Such affixing of the anchor is also done at the borehole mouth. However, to permit the extension or accommodation movement of the anchor, at least a portion of its length must be allowed free play in the borehole.
It is known to secure or safeguard the distance of free play or movement of an extensible anchor, whereby the anchor is equipped with a flexible hose-like compensating or slip tube or sleeve which extends between the fixed regions of the anchor in the base of the borehole and in the vicinity of the borehole mouth. The anchor rod is otherwise securely cemented into the borehole over the respective full length, e.g. by hydraulic cement or synthetic resin grout or similar compositions. Utilization of a slip element enhances the extensibility of the anchor in the region where the free play is permitted.
It is a disadvantage of the known anchor that it does not provide a sufficiently high reinforcement of the surrounding rock because of the nature of this sleeve and the surface configuration thereof.